What’s the difference between the three-month and “longer-term” supply items?

Three-month supply items are non-perishable foods that you normally eat, such as canned and commercially packaged foods that have a stable shelf life. “Longer-term” supply items are basic food items like grains and beans that have very low moisture content (about 10% or less), can be stored for long periods of time (20–30 years), and would sustain life if nothing else were available to eat. A portion of longer-term supply items may be rotated into the three-month supply.

From frequently asked questions at ProvidentLiving.org